The present invention relates generally to Sagnac Raman amplifiers and lasers for telecommunications, cable television (CATV), and other fiber-optics applications. More particularly, the invention relates to broadband Sagnac Raman amplifiers and lasers that have substantially improved bandwidth and noise performance.
Because of the increase in data intensive applications, the demand for bandwidth in communications has been growing tremendously. In response, the installed capacity of telecommunication systems has been increasing by an order of magnitude every three to four years since the mid 1970s. Much of this capacity increase has been supplied by optical fibers that provide a four-order-of-magnitude bandwidth enhancement over twisted-pair copper wires.
To exploit further the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunication industry: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM enables different wavelengths of light to carry different signals parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth, it would not be cost-effective without optical amplifiers. In particular, a broadband optical amplifier that permits simultaneous amplification of many WDM channels is a key enabler for utilizing the full fiber bandwidth.
With the advent of erbium-doped fiber amplifiers (EDFAs) around 1990 to replace electronic repeaters, the capacity of telecommunication systems has since been increased by almost two orders of magnitude. Although EDFAs have had a significant impact in the past five years, they are not without problems. As shown in FIG. 1a, there are two main low-loss telecommunications windows in silica-based optical fibers at wavelengths of 1.3 xcexcm and 1.55 xcexcm. EDFAs work only in the 1.55 xcexcm window. Yet, most of the terrestrial fibers installed in the United States during the 1970s and up through the mid 1980s are designed for operation at 1.3 xcexcm, and thousands of miles of 1.3 xcexcm terrestrial fibers have already been laid. This presents major difficulties in upgrading to the higher bandwidth EDFA technology. In the prior art, some have sought to combine EDFAs with dispersion compensators in an effort to correct the wavelength mismatch. However this approach does not permit further upgrading based on wavelength-division-multiplexing, and therefore is not seen as the best solution. Others are experimenting with new glass formulations that might provide the advantages of EDFAs at the shorter 1.3 xcexcm wavelength. However, currently no glass formulation has proven to be commercially viable.
Aside from the wavelength mismatch, EDFAs are also inherently prone to signal loss when the pump laser fails. EDFA is a system of the type known as a xe2x80x9cthree-levelxe2x80x9d system that does not allow the optical signal to pass through unless its pump laser is operative. Reliance on the xe2x80x9cthree-levelxe2x80x9d system could have catastrophic consequences for the reliability of fiber networks.
Stimulated Raman scattering amplifiers are advantageous over EDFAs because they can operate in both optical communication windows and, in fact, over the entire transparency window of optical fibers. Moreover, the stimulated Raman scattering amplifier is a xe2x80x9cfour-levelxe2x80x9d system that simply provides no gain when its pump laser is off, but otherwise allows the optical signal to pass through the system. Stimulated Raman scattering amplifiers are based on nonlinear polarization of the dielectric silica host, and are capable of cascading to higher Raman orders or longer wavelengths. However, there is a significant problem with Raman amplifiers that has not heretofore been really overcome. Virtually every light source or pump produces some intensity fluctuation. When Raman amplifiers are allowed to cascade through several orders, the pump source intensity fluctuations are combinatorially multiplied, and very rapidly result in enormous intensity fluctuations that have made systems virtually unusable. Compounding this problem, the gain produced by this nonlinear response is proportional to instantaneous pump intensity. Thus there is no opportunity to xe2x80x9caverage outxe2x80x9d intensity fluctuations over time. Moreover, the gain produced by Raman scattering is, itself, an exponential effect. All of these properties have lead most to conclude that stimulated Raman scattering amplifiers and cascade lasers are not suitable in general-purpose telecommunication applications.
Aside from the fluctuation problems above, several other issues also need to be addressed in order to achieve usable broadband stimulated Raman scattering amplifiers. In the prior art, a cladding-pumped fiber laser has been used as a pump source for Raman amplifiers. A commercial unit delivers 9W of single-transverse-mode output at 1100 nm with a spectral width of 4 nm. The fiber used in this laser is a rare-earth-doped, double-clad fiber. As depicted in FIG. 14a, cavity mirrors are applied to the fiber ends. The mirror applied to the input end is highly reflective at the lasing wavelength of 1100 nm, while a low-reflectivity mirror or grating is applied to the output end of the fiber. The gain band for ytterbium doped fiber is roughly between 1030 nm and 1160 nm, but using a grating at the fiber end to select one particular wavelength yields a bandwidth of about 4 nm.
While this cladding-pumped fiber laser is already quite broad in bandwidth because of multiple longitudinal modes in the cavity, it would be desirous to further broaden the pump wavelength range to achieve broadband Raman gain. The broader pump bandwidth is also advantageous to avoid reflections associated with stimulated Brillouin scattering in the gain fiber of the Raman amplifier.
Polarization controllers (PCs) are used in almost all Raman amplifiers to regulate polarization states. A fiber based PC is typically constructed using quarter-wave loops of optical single-mode fiber mounted in such a way as to allow precise rotation of the loops about a common tangential axis. Each loop is designed to function as a quarter-wave retarder for the wavelength range of interest. By rotating a loop about its tangential axis, the loop""s birefringence is rotated. Combining three or four loops in series increases the wavelength range and adjustment range of the controller and enables complete and continuous polarization adjustability. However, as the temperature changes, the fiber birefringence changes and the mechanical setting of the PCs may also be perturbed. As a result, the PCs may ruin the xe2x80x9cturn-keyxe2x80x9d operation of the amplifier because they could require periodic readjustment with changing environmental conditions.
In the past attempts at applying Raman amplifiers to analog signal amplification, it was discovered that a major limitation arises from the noise associated with Double Rayleigh Scattering (DRS). Stimulated Rayleigh scattering refers to light scattering due to induced density variations of a material system. More specifically, stimulated Rayleigh refers to the scattering of light from isobaric density fluctuations.
Stimulated Rayleigh scattering gives rise to a backward traveling wave that is at the same center frequency as the signal input, somewhat broadened by the Rayleigh linewidth (defined as a reciprocal to characteristic decay time of the isobaric density disturbances that give rise to Rayleigh scattering). For example, J. L. Gimlett, et al., IEEE Photonics Technology Letters, Vol. 2,p.211 (March 1990) disclosed that the Rayleigh scattering can be modeled as a Rayleigh mirror with a prescribed reflectivity. DRS refers to a second stimulated Rayleigh scattering event that scatters the backward traveling wave back into the original signal, thereby leading to interference with the original signal, cross-talk, and increased uncertainty of the amplitude (i.e., noise). Also, the DRS is proportional to the pump intensity, the signal intensity, and the length of the gain fiber. Therefore, the DRS noise source is a direct consequence of requiring high pump powers and long interaction lengths due to the inefficiency of the Raman amplification process.
Prior art has shown that insertion of an optical isolator midway through the amplifier and the use of two WDMs to guide the pump radiation around the isolator can reduce the DRS effect. In effect, the amplifier is split into two parts and the net gain is accumulated through both sections, but the isolator reduces the DRS in half. Although this technique has been used for high gain EDFAs and in ring designs of Raman amplifiers, it increases the complexity and cost of the amplifier considerably due to the need for two additional WDMs and one isolator.
As shown from the attenuation curve for fibers in FIG. 1a, there are two low-loss windows for telecommunications. In the prior art, EDFA technology has been developed to make full use of the 1.5 xcexcm window. Since Raman amplification can be obtained over the entire transparency range for optical fibers, Raman amplification can be applied to both the 1.3 xcexcm and 1.5 xcexcm windows. Because future communication applications will demand the broadest bandwidth available over the existing fiber base, to fully utilize optical fiber""s bandwidth, it is desirable to have an amplifier which will use both telecommunications windows and operate with WDM simultaneously.
It is an object of the present invention to provide a Sagnac Raman amplifier and cascade laser which is operable in both 1.3 xcexcm and 1.5 xcexcm windows.
It is another object of the present invention to provide a broadband Sagnac Raman amplifier and cascade laser which is operable in both 1.3 xcexcm and 1.5 xcexcm windows.
It is another object of the present invention to provide a broadband pump for use in a broadband Sagnac Raman amplifier and cascade laser.
It is another object of the present invention to remove environment-sensitive elements from the cavity of the Sagnac Raman amplifier and cascade laser.
It is another object of the present invention to provide a polarization independent Sagnac Raman amplifier and cascade laser.
It is another object of the present invention to improve noise performance of the Sagnac Raman amplifier and cascade laser.
It is yet another object of the present invention to provide a parallel optical amplification apparatus having a combination of the Sagnac Raman amplifier and EDFA for the 1.3 xcexcm and 1.5 xcexcm low-loss windows of optical fibers.
The present invention attacks the intensity fluctuation problem with Raman amplifiers by recognizing that higher order intensity fluctuations are a distributed effect (everywhere present in the distributed gain medium that produces the optical signal gain) that can be significantly reduced by a reflector structure that rejects intensity fluctuations originating in this distributed effect. The present invention employs a reflector structure that defines two optical paths within the distributed gain medium, configured to support both common mode and difference mode optical signals. By choosing a configuration that propagates higher order intensity fluctuations in the difference mode, much of the unwanted amplification of pump fluctuations is rejected.
Although numerous configurations are possible, one embodiment employs a Sagnac interferometer as one of the two optical resonator reflectors. The Sagnac interferometer employs an optical coupler with both ends of a fiber loop (a distributed gain medium) connected to its light splitting ports. The coupler thus establishes two optical paths, a clockwise path and a counterclockwise path. Signals are compared at this optical coupler, with common mode signals being substantially reflected and difference mode signals being at least partially rejected through a rejection port associated with the optical coupler. Although intensity fluctuations originating at the pump (at the pump wavelength) are amplified, any intensity fluctuations resulting from higher order stimulation of the distributed gain medium are at least partially rejected as difference mode signals.
This specification describes inventions leading to a broadband Raman amplification that would be compatible with WDM technologies. Four improvements over the original Sagnac Raman amplifier and laser are discussed.
First, broad bandwidth is achieved by using a broadband laser or amplifier cavity combined with a broadband pump. The broadband pump has a pump laser and a bandwidth adding mirror connected thereto to generate a broadened pump spectrum. The bandwidth adding mirror can be a Sagnac loop mirror with an unequal ratio coupler. It further has a phase/amplitude modulator asymmetrically located within the Sagnac loop mirror. The pump laser is a cladding pumped fiber laser. In one preferred embodiment, the broadband pump is incorporated directly into the laser or amplifier cavity.
Second, turn-key operation is obtained by minimizing the need for polarization controllers through use of a polarization maintaining cavity. In one embodiment, the Sagnac loop mirror of the broadband Sagnac Raman amplifier is fabricated from polarization maintaining fiber cross-spliced at the middle of the loop mirror. In another embodiment, the Sagnac loop mirror is made of polarization maintaining fiber and the Raman gain fiber is separated from the Sagnac loop mirror. Input and output ports of the amplifier are polarization maintaining WDMs.
Third, the noise performance is improved and protection against double Rayleigh scattering is provided by using a polarization diversity pumping system. In one embodiment of the polarization diversity pumping system, the pumping light is launched at a 45 degree angle into the polarization maintaining fiber to produce a beam having two polarization directions. Such angle is achieved by either rotating the fiber or using a quarter wavelength plate. In another embodiment, the pumping light is first divided by a 50:50 coupler into two beams. One beam travels through a retarder to change its polarization direction. Then a polarization beam splitter combines the two beams In yet another embodiment, the polarization maintaining fiber is spliced at a 45 degree angle to the cladding-pumped fiber to output a beam having two polarization directions.
Finally, two-wavelength operation is achieved between two parallel amplifiers for two separate windows. In one embodiment, both 1310 nm and 1550 nm amplifications are performed by the broadband Sagnac Raman amplifiers. Moreover, the two amplifiers share a common pump laser. In another embodiment, a combination of Raman amplifiers and EDFAs are used. The Sagnac Raman amplifier is used to amplify the 1310 nm signal, while the 1550 nm signal is amplified by the EDFA. The EDFA may be pumped by another Sagnac Raman cascade laser.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.